If, in conventional fuel amount setting arrangements for internal combustion engines, the load signal fails, this is detected by a means which tests the presence of this signal and in response, the means switches over from normal mode to emergency operation mode. In the emergency operation mode, a plurality of permanently predetermined emergency injection times are used in dependence upon certain operating states. Since the necessary fuel requirement, however, depends to a large degree on the load, the same amount of fuel at, for example, a certain engine speed leads to widely varying lambda values in dependence upon the operating conditions present in each case. The error in the fuel metering can be so large that the intake air/fuel mixture can no longer be ignited. Thus, a large degree of environmental damage may arise due to fuel which has not been combusted at all or only incompletely combusted. In addition, the catalytic converter present in lambda-controlled systems is damaged if it has reached its working temperature and a non-combusted mixture reaches it.
Japanese patent publication 59 028 030 describes a process which permits the emergency operation of an internal combustion engine in the event of the failure of the air-flow sensor. Within the scope of this process, only a basic injection time is used for metering the amount of fuel. This basic injection time is essentially multiplied by the reciprocal value of the engine speed and, in addition, contains signals of an idling switch and of an air/fuel ratio sensor.
A process for lambda control, which, even in normal operation, has no special sensor for detecting a load signal is disclosed in German patent publication DE-OS 3,714,245. In the process described, the detection of a load signal (air-flow measuring component) is dispensed with in order to reduce the complexity of the control arrangement. The fuel metering signal is formed as a function of the difference of a lambda actual value and of a lambda desired value. The lambda desired value is taken from a memory from a stored characteristic field as a function of the engine speed and an air-flow value. The air-flow value is, in turn, computed from the lambda actual value and the fed-back fuel metering signal.